Readout circuit and method for sparse readout of active and neighboring pixels in a multi-pixel sensor array

ABSTRACT

A readout circuit ( 29 ) for reading active pixels in a sensor ( 10 ) having at least one sensor segment ( 11 ) each containing addressable pixels ( 12 ) includes a respective sample and hold unit ( 28 ) for sampling and holding an analog value associated with a corresponding pixel, and an analog multiplexer ( 35 ) in each segment having an addressable channel coupled to each sample and hold unit for carrying the corresponding held value of the respective pixel. A lookup table ( 38 ) stores addresses of predefined neighboring pixels associated with the respective pixel. An encoder ( 36 ) is responsive to one or more trigger signals for generating an address in the lookup table, and a controller ( 37 ) feeds the address to the lookup table and successively feeds the addresses of the predefined neighboring pixels output by the lookup table to the analog multiplexer.

FIELD OF THE INVENTION

This invention relates to a charge detector for reading charge producedby an active pixel in the detector.

BACKGROUND OF THE INVENTION

A known diagnostic technique used in tomography for locating tumorsinvolves injecting into a patient's bloodstream a radioactive isotopewhich targets the tumor, so that the location of the tumor can bederived by detecting the location of the radioactive isotope. Typically,the radioactive isotope emits γ-rays which are dispersed from the tumorsite. In order to achieve the desired detection so as to determine theprecise location of the tumor, it is necessary to image the patient'sbody in such a manner as to detect only those γ-rays which are emittednormally from the body and to ignore those γ-rays which are dispersed inother directions.

Different types of computer tomography are known in which such aradiation imaging system may be embodied. In Single Photon EmissionComputed Tomography (SPECT) more than one detector is rotated around thesubject and the radioisotope's distribution (tomographic image) isreconstructed based on an obtained count values of the γ-rays.

In contrast to SPECT where a radioisotope in the body emits γ-raysproduced by a single photon, in Positron Emission Tomography (PET) apatient is administered a radioisotope that emits positrons (i.e.positively charged electrons). When the positrons meet electrons withinthe body, the positrons and electrons mutually annihilate and producetwo γ-rays that propagate away from each other at an angle of 180° andare detected by respective detector segments in the PET scanner. Thescanner's readout electronics record the detected γ-rays and map animage of the area where the radioisotope is located. Here also twosimultaneous detections are indicative of a positron emission from thetumor site.

Ideally, if during every scan of the composite image sensor, each pixelis read sequentially only one at a time, then the current scan in eachsegment can be terminated when an “active” pixel is detected assumingthat only pixel in each segment can be active. However, two factorsmilitate against this ideal approach. First, it is impractical to readeach pixel in such a manner because of the time overhead involved inaddressing each pixel separately and downloading the pixel data along adedicated channel for further processing. Secondly, this ideal approachassumes that each pixel corresponds to a strike by single photon.However, in practice, the energy of an impinging γ-ray may not betotally absorbed by a single pixel but may be shared by more than onepixel. This will occur, for example, when the γ-ray strikes a pixeloff-center so that its energy is shared by a central pixel (usuallyabsorbing most of the energy) and one or more neighboring pixels, whichtogether absorb the residual energy. It can also occur owing to Comptonscattering, which may occur in any high-energy particle detector asexplained above but forms the principle of operation in a Comptoncamera.

For the purpose of the present discussion, a Compton camera may beregarded as just another type of 2-dimensional image sensor having aplurality of addressable pixels, one of which emits a signal whenstimulated by a γ-ray. Specifically, each pixel is a diode whichgenerates a charge signal when hit by a γ-ray. A γ-ray emitted by theradioisotope will be detected only if it creates a Compton effect bycreating a charge signal thereby giving up some of its energy. Inpractice, it is usual to employ a composite sensor having severalspaced-apart sensor layers each containing at least one sensor module soas to increase the probability that an incident γ-ray will produce aCompton effect in at least one of the layers. The multi-layer sensormodule constitutes a first detector of the Compton camera. Having thusproduced a Compton effect, the γ-ray then emerges from the firstdetector. However, in order to calculate the angle of the incidentγ-ray, the emergent γ-ray is directed to a second detector in which itis absorbed completely, thereby giving up all of its residual energy.Such a detector is described in EP 893 705 published on Jan. 27, 1999entitled “Multi-Channel Readout circuit for Particle Detector” andassigned to the present applicant.

Photons produced by Compton scattering are formed substantiallysimultaneously. The scattering process by which this typically occurs isthat the photon interacts with an electron (primary hit), but not all ofits energy is deposited. The photon changes direction, and may depositthe remainder of the energy in a different pixel with a secondaryinteraction with an electron. Thus, if two active pixels are detectedsimultaneously and their combined energy is conducive with their beingderived by Compton scattering from an incident γ-ray having an energy of511 keV, then the position of each energy emission allows a collisionline between the locations of the first and second collisions to bedetermined, and their respective energies allow determination of theangle of Compton scattering.

It is thus desirable to read two or more active pixels in a pixel arraywithout the need to read all the pixel data so as to reduce the timerequired to perform computer tomography and hence the time for which apatient is exposed to radiation.

One known approach to doing this is to use so-called sparse readout,such as described in U.S. Pat. No. 5,847,396 (Lingren et al.) assignedto Digirad Corporation, which discloses a high-energy photon imagingsystem comprising an imaging head that includes a detector having aplurality of detection modules. Each detection module comprises aplurality of detection elements fixed to a circuit carrier. The circuitcarrier includes channels for conditioning and processing the signalsgenerated by corresponding detection elements. Each channel stores theamplitudes of the detection element electrical pulses exceeding apredetermined threshold. The detection modules employ a fall-throughcircuit, which avoids the need for sequential readout and automaticallyfinds only those detection elements whose stored pulse amplitude exceedsthe threshold. The fall-through circuit searches for the next detectionelement and associated channel having a valid event, meaning that thedetection element exhibits a pulse magnitude that exceeds a certainthreshold. Use of sparse readout in a PET camera is also described inU.S. patent application Ser. No. 09/827,439 filed Apr. 5, 2001 in thename of the present assignee and entitled “Improved Readout circuit fora Charge Detector”.

Another approach that may be used independently of sparse readout, or inaddition thereto, is described in U.S. Pat. No. 5,825,033 (Barrett etal.) published Oct. 20, 1998 and entitled “Signal processing method forgamma-ray semiconductor sensor”, which relates to the need to readsub-threshold data of neighboring pixels. In order to do so, all pixelsmust be read out in order to define a “central pixel” and a“neighborhood” of related neighboring pixels whose data must be also beread since there exists an a priori likelihood that charge is sharedbetween the central pixel and one or more of the neighboring pixels.Specifically, it is to be noted that the voltage signal for each pixelmust be compared to a corresponding predetermined threshold in order toidentify all pixels having an above-threshold voltage signal.Thereafter, clusters of adjacent pixels are identified havingabove-threshold voltage signals and a cumulative voltage signalassociated with the cluster is calculated. Thus, U.S. Pat. No. 5,825,033is not related to a sparse readout system that attempts to reduce thenumber of pixels whose voltage signals must be read, but rather readsall pixels in the pixel array.

U.S. Pat. No. 5,107,122 (Barkan et al.) published Apr. 21, 1992 andentitled “Sparse readout method and apparatus for a pixel array” doesrelate to sparse readout of a pixel array where outputs are obtainedonly from selected pixels. These pixels are determined by those pixelsthat have received actuating inputs, and may consist of the hit pixelsand their immediate neighbors. The system obtains outputs from theselected pixels only for the times that correspond to the occurrence ofan event of interest. By such means, the amount of data to be processedis substantially reduced. A content addressable memory is used to storethe times when photons strike pixels, whose locations in the pixel arrayare stored in a random access memory. Active pixels are read sparselycolumn-by-column and on reaching an active pixel, the pixel in each rowin the active column for which a hit occurred since the time of theprevious event of interest is accessed and compared with the rowaddresses of the pixels hit at the time of the current event ofinterest. By such means associated pixels may be read being those thatare hit at the same time as the current event of interest. However,these pixels must be in the same pixel array and thus the readout methodis suitable for charge sharing but may not be suitable for Comptonscattering where charge can be scattered between spatially separatedpixel arrays.

It is thus to be noted that neighboring pixels are determined on the flyaccording to the time stamp associated with each event. This allowsassociated pixels to be determined on the basis of simultaneity ofphoton impingement. Although sparse readout is used to identify activecolumns and thus avoids the need to read each pixel sequentially, thereis no attempt to define a priori for each pixel a subset of neighboringpixels, being those most likely to be associated with an active pixel.

Another problem in all readout circuits relates to dead time in thesystem. When reading out one pixel in a sampled system, all pixels thatare sampled but not read are also “dead”. As such, if during the act ofsampling these pixels, a different photon strikes one of the sampledpixels, this event will be lost. It is impossible to eliminate dead timealtogether but by sampling only a subset of the pixels, the other pixelsshould in principle be in an active “reading-mode” and so the effects ofdead time are reduced. This is true also in U.S. Pat. No. 5,107,122 butonly for the single detector segment to which this patent relates. Thus,it is clear from FIG. 2 of U.S. Pat. No. 5,107,122 that only a singledetector segment is contemplated. Moreover, it is to be noted that thereadout chip in U.S. Pat. No. 5,107,122 is configured as an array havinga plurality of readout circuits each corresponding to a respective pixelin the sensor chip and being aligned therewith and connected thereto bymeans of bump contacts. Such an arrangement is particularly convenientfor a single detector segment, since neighboring pixels in the sensorchip are directly mapped to neighboring circuits in the readout chip.

However, in practice, it is not always feasible to employ such astructure. For example, in our EP 893 705 there are disclosed multipledetector segments each comprising an array of 16×16 pixels, i.e. 256pixels per pixel array and each being coupled to a corresponding channelof a one-dimensional ASIC having 256 channels. Each ASIC channelprovides pre-amplification, noise-filtering and generation of triggersignals. A trigger is generated whenever the input charge exceeds acertain threshold. If the primary hit alone is detected by the ASIC, theremainder of the charge deposited in other pixels can be recovered byreading out the neighboring pixels. In a one-dimensional detector array,neighboring detector elements are usually connected to neighboring ASICchannels. Reconstruction of physical events in case of charge sharing orCompton scattering can be done by reading out neighboring ASIC channels.However, in a system where pixels are connected to an ASIC having aone-dimensional channel array structure, the two dimensions of the pixelarray are mapped into one dimension.

It thus emerges that the simple structure of U.S. Pat. No. 5,107,122allowing direct mapping of neighboring pixels to neighboring circuitelements in the readout chip is impractical. When mapping atwo-dimensional sensor to a one-dimensional ASIC, neighboring pixels,such as those along an edge of the sensor chip that border an activepixel in a different row or column, may map to a channel in the ASICthat is remote from the channel associated with the active pixel. Thisrenders impossible an intuitive understanding as to which channels inthe ASIC neighbor the channel associated with the active pixel.

This problem is, of course, exacerbated when multiple detector segmentsare used. In computer tomography applications, multiple detectorsegments may be required in order to increase the effective area thatcan be imaged. Moreover, in Positron Emission Tomography (PET) a patientis administered a radioisotope that emits positrons (i.e. positivelycharged electrons). When the positrons meet electrons within thepatient's body, the positrons and electrons mutually annihilate andproduce two γ-rays that propagate away from each other at an angle of180° and are detected by respective detector segments in the PETscanner. Thus, two detectors are required on opposite sides of thepatient. Each of these detectors may, and typically will, comprisemultiple sensor segments.

Moreover, when multiple sensor segments are used, potential neighboringpixels may in fact reside in adjacent sensor segments, thus requiringthat one or more adjacent sensor segments be sampled. Such sensorsegments are typically “dead” for the read out period such that newtrigger events are lost. This applies regardless of whether the sensorsegments are part of the same detector or belong to different detectors.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a readout circuitfor reading an “active” pixel in an image sensor having a plurality ofpixels and for each active pixel reading neighboring pixels that aredefined for the active pixel a priori and thus may be read out withoutreading other pixels in the pixel array.

According to a broad aspect of the invention there is provided a methodfor sparsely reading data representative of pixel energy of an activepixel and of neighboring pixels in at least one sensor segment having aplurality of addressable pixels, the method comprising:

-   -   (a) for each pixel in the at least one sensor segment storing in        a lookup table addresses of predefined neighboring pixels,    -   (b) on determining that a pixel in the at least one sensor        segment is active, using an address of said pixel to read from        the lookup table corresponding addresses of the neighboring        pixels associated therewith, and    -   (c) reading data representative of pixel energy of the active        pixel and of successive ones of its neighboring pixels.

A readout circuit in accordance with the invention for reading activepixels in a sensor having at least one sensor segment each containing aplurality of addressable pixels, comprises:

a sampling circuit coupled to each pixel in each of the segments, forsampling an energy level associated with at least one active pixel,

a lookup table having a plurality of addressable locations eachcorresponding to a respective pixel in the sensor and storing addressesof predefined neighboring pixels associated with the respective pixel,

an encoder having a plurality of input lines each for connecting to arespective trigger channel corresponding to each pixel in the sensor andresponsive to one or more trigger signals for generating an address insaid lookup table, and

a controller coupled to an output of the encoder for feeding the addressgenerated by encoder to the lookup table and for feeding the addressesof the predefined neighboring pixels output by the lookup table to arespective channel of the sampling circuit for reading the energy levelof the respective neighboring pixel.

Thus, according to the invention, a “center” or “primary” pixel is firstdefined and this allows automatic determination of the neighboringpixels. Only after deriving this information, is the pixel energy levelread-out. This results in a much sparser readout than hitherto proposedapproaches. More specifically, this is so because in the invention thethreshold detection is done before the multiplexing. In addition, theread-out can be terminated before the entire neighborhood has been readout, if the controller determines that all of the charge has beencollected. The architecture supports one or several signals abovethreshold. In principle, all channels can actually receive a trigger andstill be read out. In a preferred architecture, typically both theenergy and the address of the primary hit are read out simultaneously.

Traditionally, a low threshold is needed in order to identify alldetector elements that receive part of the charge, as the triggers areused to flag which pixels should be read out. The primary pixel isusually easy to detect, as most of the energy is deposited in thisdetector element. Providing technical solutions for a threshold that islow enough for all detector elements that contain a minor part of thecharge to be detected is difficult, as the noise limit of the systemimposes a lower limit. Thus, if the threshold is set too low, thenspurious noise signals will be registered as active pixels and thisimposes a minimum threshold, which may be too high to trap secondarypixels that received a minor portion of the charge. The inventionprovides a solution for identifying all energy information, based on adetection of the primary hit alone. This allows the threshold to be kepthigh, which simplifies the design significantly.

The invention is applicable to a pixel sensor having one or moredimensions and thus contemplates also one-dimensional pixel arraystructures. However, the readout circuit according to the inventionspecifically addresses the mapping from a two-dimensional pixel array toone or more one-dimensional ASICs (column of read-out channels).

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnonlimiting example only, with reference to the accompanying drawings,in which:

FIG. 1 is a schematic diagram showing a partial detail of a particledetector;

FIG. 2 is a schematic diagram showing part of a readout circuitaccording to the invention for reading the energy values associated withsimultaneously active pixels in the particle detector of FIG. 1; and

FIG. 3 is a flow diagram showing a method for reading a primary pixeland associated neighboring pixels using the readout circuit of FIG. 2.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 shows schematically a partial detail of a particle detectordepicted generally as 10 comprising one or more sensor segments 11 eachhaving a plurality of detector elements or pixels 12, only two pixelsand their respective channels being shown in the figure. All pixels 12are connected to respective preamplifiers 20 whose outputs are fed to arespective fast shaper 21 for establishing an incident time of radiationstriking the pixel and to a respective slow shaper 22 for determiningthe peak energy value of the pixel if it goes active. The respectiveoutput of each fast shaper is fed to a threshold discriminator 23 whoseoutput is fed to a monostable 24 whose output goes high when the outputof the fast shaper exceeds a predetermined threshold level 25, andremains high until the monostable 24 is reset The outputs of the slowshapers 22 are fed via a respective buffer 26 to a respectivetrack-and-hold circuit 27 whose output tracks closely the output of theslow shaper 22, but only as long as the amplitude increases (forpositive input signals). When the amplitude of the output of the slowshaper 22 sinks after it has reached its peak, the output of thetrack-and-hold circuit 27 stays constant at the maximum amplitude untilbeing reset. By such means, the respective pixel energy of all pixels islatched regardless of whether the pixel energy is below or abovethreshold. This feature makes it possible to delay sampling until afterthe center pixel has been read out, which gives the benefit of reduceddead-time. As shown in FIG. 1 the reset signal is fed directly to theanalog part of the channel constituted by the track-and-hold circuit 27,as well as to the trigger part of the channel constituted by themonostable 24.

The maximum signal level held by the track-and-hold circuit 27 is readby a respective sample and hold circuit 28 if activated by an externalsample and hold signal produced by a readout circuit 29. The monostables24 constitute trigger outputs that are input to the readout circuit 29.The outputs of the sample and hold circuits 28 are fed via a buffer 30to the readout circuit 29. Typically, the preamplifiers 20, the fast andslow shapers 21 and 22, the threshold discriminators 23, the monostables24, the buffer 26 and the track-and-hold circuits 27 are constituted bya corresponding channel of an ASIC 31 having a plurality of channels,and which may also integrate some of the components described below withreference to FIG. 2 of the drawings. The ASIC 31 thus has a plurality ofchannels, albeit possibly fewer than the total number of pixels 12 inthe sensor segments 11, which would then require multiple ASICs eachconnected to a respective group of pixels in each sensor segments 11.

FIG. 2 is a schematic diagram showing the readout circuit 29 for readingthe energy values associated with simultaneously active pixels in theparticle detector 10. The amplified and noise-filtered energy-signaloutput from the ASIC channels are denoted E₁–E₂ _(N) . These channeloutputs are connected to the inputs of a 2^(N):1 multiplexer 35. Thetrigger outputs generated by the respective monostables 25 in each ofthe ASIC channels (T₁–T₂ _(N) ) are connected to the inputs of a 2^(N):Nencoder 36. The N-bit output of the encoder 36 is connected to theinputs of a controller 37. When a signal exceeds the threshold, thecorresponding ASIC channel generates a trigger. This trigger generates aN-bit address on the output bus of the encoder 36. This can be doneeither asynchronously or hand-shaken. The controller 37 is operativelycoupled to a look-up table 38 containing the addresses of the channelsthat correspond to the neighboring pixels of the primary hit pixel.Thus, it performs the mapping from the two-dimensional detector pixelarray on to one or more of the one-dimensional ASIC channel arrays. Anoutput bus of the controller 37 is connected to a N:2^(N) decoder. Theoutputs of the decoder are connected to the N control inputs of themultiplexer 35. The input selected by the control inputs is connected tothe output of the multiplexer 35, thus allowing the corresponding analogenergy value of the addressed pixel to read out. The energy of theneighboring channels is read out by addressing them sequentially on theoutput bus of the controller 37, thus reconstructing a physical event ofsimultaneous impacts of multiple pixels by a single photon by detectingonly the primary hit pixel.

Preferably, the controller 37 further includes an accumulator 39 forcalculating cumulative energy read from the active pixels and successiveneighboring pixels as the respective pixel energy of each successiveneighboring pixel is read. A reset circuit 40 is coupled to theaccumulator 39 and is responsive to the cumulative energy exceeding apredetermined value for resetting the encoder, typically under controlof the controller 37 to which a reset signal is fed. When used in aparticle detector for detecting γ-rays, the energy of each γ-ray is 511keV, which may be shared by a primary pixel and associated neighboringpixels, as explained above. The reset circuit 40 allows the reading ofsuccessive neighboring pixels to be terminated in the event that thecumulative energy of those pixels read so far equals or is close to 511keV, thus obviating the need to read remaining neighboring pixels whichin any case will be inactive.

It should be noted that the ASIC 31 may also include all or part of theadditional circuitry described above with respect to FIG. 2 and, in anactual design reduced to practice, the ASIC 31 included the multiplexer35 and the encoder 36.

In use, a preferred implementation employs two read-out modes. The firstmode will be used to read out center pixels. This read-out mode couldfor instance be as described in our above-mentioned U.S. patentapplication Ser. No. 09/827,439 or in U.S. Pat. No. 5,847,396 alsomentioned above. Thus, in the first pass, only channels with triggersare read out. Based on this, the controller 37 can (possibly with somehelp from the information stored in the look-up table 38) define whichneighbors to read, if any. The controller 37 then sets the ASIC 31 tothe second mode, which reads out the neighboring pixels addressed by thelook-up table 38. Evident from this, the architecture supports inprinciple any number of simultaneously triggered channels, and anycombination of pixels containing all or part of the charge.

FIG. 3 is a flow diagram showing how charge distributed between aprimary (or “active”) pixel and one or more neighboring pixels may beread using the readout circuit 29, without needing to obtain triggeroutputs from the neighboring pixels. Thus, for each pixel in each sensorsegment there is stored in the lookup table 38 addresses of predefinedneighboring pixels. On determining that a pixel in the at least onesensor segment is active, the address of the active pixel is used toread from the lookup table 38 corresponding addresses of the neighboringpixels associated therewith. Thus, addresses of the neighboring pixelsare successively read from the lookup table 38 and fed to themultiplexer 35. By such means, data representative of the pixel energyof the active pixel and of its neighboring pixels are successively readfrom the analog multiplexer.

Sample and hold (S/H) and reset can be performed in the followingmanner. Triggered channels sample themselves, and are reset afterread-out. Pixels which are read out using the address mechanism can besampled upon addressing and reset after read-out. It should be notedthat the sampling (addressing of the neighboring pixels) and theeventual read-out can be done in any order. For instance, all pixelscould be sampled first and then read out or each pixel could be sampledand read out before the next pixel.

As the energy of each pixel is read, the cumulative energy associatedwith the primary pixel is calculated, and the energy of each successiveneighboring pixel is read for so long as the cumulative energy is lessthan the total energy associated with the photon. For example, in aparticle detector using γ-rays whose energy is 511 keV, a threshold maybe set close to this value so that when all, or substantially all, of anincident photon's energy is accounted for, any remaining neighboringpixels need not be read, since they will in any case have no sharedphoton energy.

The manner in which neighboring pixels are assigned to each pixel in thesensor may vary according to each specific application. In a simpletwo-dimensional pixel array, a 3×3 or 5×5 sub-matrix centered on theprimary pixel is usually sufficient to retrieve the exact position andenergy of a photon bombardment. Thus, the addresses of all nine pixelsin a 3×3 matrix centered on each pixel, or of all 25 pixels in a 5×5sub-matrix centered on each pixel are stored in the lookup table 38. Tooptimize the read-out speed, in a 3×3 sub-matrix with the triggeredpixel being in the middle, four of the immediate neighbors will have ahigher chance for sharing parts of the charge. This is easily understoodsince the four corner pixels in the sub-matrix share only a corner withthe primary pixel, as distinct from the four remaining neighboringpixels which abut the active pixel along a complete edge thereof. Thus,the likelihood is lower for charge sharing to occur with the four cornerpixels. The sequence of the neighboring pixels in the first row (and tosome extent also the second row) can thus be optimized.

However, the principles of the invention are not limited to simpleplanar structures. Thus, the invention is equally applicable in aCompton camera similar to that described in EP 893 705, where a sensorcontains several spatially separated sensor segments. In such a sensor,a photon striking one sensor segment can be Compton scattered in asecond segment. Theoretically, Compton scattering can even occur in morethan two sensor segments, although statistically the likelihood of thisoccurring is reduced as a photon that is partially absorbed in onesegment strikes other segments displaced therefrom. It thus emerges thatthe greatest likelihood of Compton scattering is in the same segment,and the likelihood of Compton scattering becomes progressively smallerin successive displaced segments.

Thus, in accordance with empirical data, it may be assumed that Comptonscattering is limited to within two pixels from the primary pixel, inall directions. This allows neighboring pixels to be defined a priorifor each pixel for which there is a reasonable likelihood of Comptonscattering. Some of these neighboring pixels will be in the same sensorsegment and some will be in an immediately adjacent segment of amulti-segment sensor.

In a specific application reduced to practice, neighboring pixels weredefined on the assumption that Compton scattering can cross pixel arrayborders while charge sharing cannot. By identifying a primary hit on theborder between sensor segments, pixels in both sensor segments may beread out in order to reconstruct the event. From the point of view ofthe readout circuit, any pixel can be defined as a neighboring pixel andread out. But in practice it is desirable to minimize the necessarynumber of pixels to read out and therefore the manner in whichneighboring pixels are defined will inevitably be a compromise betweenthe need to account for charge sharing as accurately as possible and thedesire to do so as quickly as possible.

It will be apparent to those skilled in the art that, while a preferredembodiment has been described, modifications can be effected theretowithout departing from the scope of the invention as defined in theclaims. For example, FIG. 2 shows a particularly simple implementationof the invention only, wherein channel triggers will immediately andasynchronously cause an address to be conveyed to the controller. Thisaddress acts like an interrupt that starts a read-out sequence in whichall interesting pixels (including the center pixel) are read out bysending the primary pixel address to the look-up table.

It should also be noted that only those parts of the system relating toreading of the neighboring pixels are shown in the figures. In practice,the trigger circuitry shown in FIG. 1 may be interfaced to the readoutcircuit 29, of which only part is shown in FIG. 2. Such interface may,for example, allow for sparse readout as described in U.S. patentapplication Ser. No. 09/827,439 but this is not essential to anunderstanding of the invention. It should also be understood that theanalog multiplexer reduces hardware complexity, but is not absolutelyessential. Thus, in FIG. 2 where the energy of the active pixels is fedto the controller and accumulator by only a single connectionmultiplexing is clearly necessary. However, the invention could berealized by converting the analog signal produced by each pixel to anequivalent digital signal and then processing the digital signals. Also,theoretically, albeit not currently practically, the outputs of allchannels could be processed directly. Likewise, the pixels could bearranged in groups each adapted for processing by a respectivemultiplexer and allowing direct processing of all channels in eachgroup, thus allowing some direct and some parallel processing.

It should also be noted that while in the preferred embodiment a sampleand hold unit is used to latch the energy of each active pixel, otherapproaches are known in the art. Thus, sample and hold units are used inEP 893 705 where the trigger event is determined independently of theenergy level of the active pixel and the energy is therefore read outonly after establishing which pixels are active. This requires that thepixel energy be shaped by fast and slow shapers. However, it is known inthe art to read a pixel digitally by converting the analog pixel energyto a digital signal and then processing the digital signals. This wouldobviate the need to provide a hardware sample and hold mechanism sincethe sampling and processing could then be implemented using software.This is particularly applicable where coincidence of two or more activepixels is not processed in real time.

It should also be noted that the track-and-hold component 27 is strictlyonly essential for latching pixel energy when pixels with sub-thresholdenergy are to be sampled at a random time after the event occurs. It isalso possible, but generally less convenient, to sample each pixel'senergy with a sample and hold unit only, providing that it is known whento sample the peak of the slow shaper.

As noted above, when addressed read-out is performed, the informationfrom the look-up table may be used as direct control of which pixelenergy is coupled to the energy output. This means that there mayactually be two entirely different read-out controls, depending onwhether the read-out is for the primary pixel or for the neighbors.According to yet another approach, all information from the look-uptable can be buffered in the channels. This is particularly beneficialfor allowing multiple trigger events, since each triggered pixel and itsrespective neighboring pixels can then be processed. If buffering is notprovided, then, as noted above, during the act of reading theneighboring pixels of a triggered event, subsequent triggered events arelost owing to “dead time”. Buffering may be done, for example, incombination with the sparse read-out of U.S. patent application Ser. No.09/827,439, which works by latching trigger information in a flag ineach channel, and reading out only flagged channels. This can also bedone in this case for the addressed read-out such that for each addressthat the look-up table supplies to the ASIC, the corresponding channelflag can be set. In this manner, all addressed channels will be read outusing the same type of control as the primary hits.

1. A method for sparsely reading data representative of pixel energy ofan active pixel and of neighboring pixels in at least one sensor segmenthaving a plurality of addressable pixels, the method comprising: (a) foreach pixel in the at least one sensor segment storing in a lookup tableaddresses of predefined neighboring pixels, (b) on determining that apixel in the at least one sensor segment is active, using an address ofsaid pixel to read from the lookup table corresponding addresses of theneighboring pixels associated therewith; (c) reading data representativeof pixel energy of the active pixel and of successive ones of itsneighboring pixels; (d) calculating cumulative energy read from theactive pixels and successive neighboring pixels as the respective pixelenergy of each successive neighboring pixel is read; and (e) repeating(c) and (d) in respect of successive neighboring pixels only for as longas the cumulative energy is less than a predetermined value.
 2. Themethod according to claim 1, wherein the addresses of the predefinedneighboring pixels associated with an active pixel correspond to pixelsin more than one sensor segment.
 3. A readout circuit for reading activepixels in a sensor having at least one sensor segment each containing aplurality of addressable pixels, the readout circuit comprising: asampling circuit coupled to each pixel in each of the segments, forsampling an energy level associated with at least one active pixel, alookup table having a plurality of addressable locations eachcorresponding to a respective pixel in the sensor and storing addressesof predefined neighboring pixels associated with the respective pixel,an encoder having a plurality of input lines each for connecting to arespective trigger channel corresponding to each pixel in the sensor andresponsive to one or more trigger signals for generating an address insaid lookup table, a controller coupled to an output of the encoder forfeeding the address generated by encoder to the lookup table and forfeeding the addresses of the predefined neighboring pixels output by thelookup table to a respective channel of the sampling circuit for readingthe energy level of the respective neighboring pixel, a thresholddiscriminator responsive to a pixel energy level for producing an outputsignal when the pixel energy level exceeds a threshold value, aresettable monostable responsive to the output signal of the thresholddiscriminator for generating said trigger signal, an accumulator forcalculating cumulative energy read from the active pixels and successiveneighboring pixels as the respective pixel energy of each successiveneighboring pixel is read, and a reset circuit coupled to theaccumulator and being responsive to the cumulative energy exceeding apredetermined value for resetting the encoder and the monostables. 4.The readout circuit according to claim 3, wherein: the sampling circuitincludes a sample and hold unit in respect of each pixel in the sensorfor carrying a corresponding sampled and held value of the respectivepixel, and there is provided in each segment an analog multiplexerhaving a plurality of addressable channels each coupled to a respectiveone of the sample and hold units for carrying the corresponding sampledand held value of the respective pixel.
 5. The readout circuit accordingto claim 3, wherein: the sampling circuit includes a track and hold unitin respect of each pixel in the sensor for carrying a corresponding peakenergy value of the respective pixel, and there is provided in eachsegment an analog multiplexer having a plurality of addressable channelseach coupled to a respective one of the track and hold units forcarrying the corresponding peak energy value of the respective pixel. 6.The readout circuit according to claim 4, wherein the controller isadapted to feed the addresses of the predefined neighboring pixelsoutput by the lookup table successively to the analog multiplexer. 7.The readout circuit according to claim 3, wherein: the sensor includesat least first and second spatially separated sensor segments, and thelookup table stores addresses of pixels in the second sensor segment inrespect of one or more pixels in the first sensor segment.
 8. Thereadout circuit according to claim 5, wherein the controller is adaptedto feed the addresses of the predefined neighboring pixels output by thelookup table successively to the analog multiplexer.
 9. The readoutcircuit according to claim 4, wherein: the sensor includes at leastfirst and second spatially separated sensor segments, and the lookuptable stores addresses of pixels in the second sensor segment in respectof one or more pixels in the first sensor segment.
 10. A charge readoutdetector having at least one sensor segment each containing a pluralityof addressable pixels, the charge readout detector including a readoutcircuit for reading active pixels, the readout circuit comprising: asampling circuit coupled to each pixel in each of the segments, forsampling an energy level associated with at least one active pixel, alookup table having a plurality of addressable locations eachcorresponding to a respective pixel in the sensor and storing addressesof predefined neighboring pixels associated with the respective pixel,an encoder having a plurality of input lines each for connecting to arespective trigger channel corresponding to each pixel in the sensor andresponsive to one or more trigger signals for generating an address insaid lookup table, a controller coupled to an output of the encoder forfeeding the address generated by encoder to the lookup table and forfeeding the addresses of the predefined neighboring pixels output by thelookup table to a respective channel of the sampling circuit for readingthe energy level of the respective neighboring pixel, a thresholddiscriminator responsive to a pixel energy level for producing an outputsignal when the pixel energy level exceeds a threshold value, aresettable monostable responsive to the output signal of the thresholddiscriminator for generating said trigger signal, an accumulator forcalculating cumulative energy read from the active pixels and successiveneighboring pixels as the respective pixel energy of each successiveneighboring pixel is read, and a reset circuit coupled to theaccumulator and being responsive to the cumulative energy exceeding apredetermined value for resetting the encoder and the monostables. 11.The charge readout detector according to claim 10, wherein: the samplingcircuit includes a sample and hold unit in respect of each pixel in thesensor for carrying a corresponding sampled and held value of therespective pixel, and there is provided in each segment an analogmultiplexer having a plurality of addressable channels each coupled to arespective one of the sample and hold units for carrying thecorresponding sampled and held value of the respective pixel.
 12. Thecharge readout detector according to claim 10, wherein: the samplingcircuit includes a track and hold unit in respect of each pixel in thesensor for carrying a corresponding peak energy value of the respectivepixel, and there is provided in each segment an analog multiplexerhaving a plurality of addressable channels each coupled to a respectiveone of the track and hold units for carrying the corresponding peakenergy value of the respective pixel.